The present invention relates to a method and apparatus for multiple sheet detection of sheets to be fed to a sheet-fed press.
A multiple sheet detection apparatus is used to perform multiple sheet detection when sheets (paper) are supplied to a conventional sheet-fed press.
More specifically, when a sheet is to be supplied from a sheet feeder to a sheet-fed press (to be referred to as a printing press hereinafter), a leading edge of a sheet is brought into contact with a stopper called a front gauge arranged at the press-side distal end portion of a feeder board, and then the sheet is gripped by a swing gripper and fed to the printing press. In this case, a multiple sheet detection apparatus is used to prevent simultaneously feeding of two or more sheets. More specifically, when two or more sheets are simultaneously fed to a printing press, a printing pressure is increased, a plate cylinder and a blanket wound around a blanket cylinder in the printing press may be deformed, and the respective cylinders may be damaged. In order to prevent this, multiple sheet detection of sheets is performed at a predetermined feed operation timing (to be described later) prior to the start of sheet feeding. When simultaneous feeding of two or more sheets is detected, the sheet feeder is stopped, and feeding of sheets to the printing press is stopped.
Conventional multiple sheet detection apparatuses are exemplified by contact (thickness detection by means of a roller), light-transmitting, ultrasonic, capacitance type multiple sheet detection apparatuses. These multiple sheet detection apparatuses are arranged in the paper feed units (or register portions) in front of the printing presses, and multiple sheet detection is performed at the predetermined feed operation timings.
For example, in the light-transmitting type multiple sheet detection apparatus, a light-emitting element is arranged on the lower surface of the feeder board near the front gauge, a through hole is formed at a predetermined portion of the feeder board opposite to a light-emitting portion of the light-emitting element, and a light-receiving element is arranged on the upper surface of the feed board at a position opposite to this through hole. More specifically, light emitted from the light-emitting element is projected in a direction of thickness of a sheet to be fed, light transmitted through the sheet is received by the light-receiving element, and an output from the light-receiving element is converted into an electrical signal. An output level corresponding to a light amount represented by the electrical signal is compared with a predetermined discrimination level, thereby performing multiple sheet detection on the basis of a comparison result. The multiple sheet detection timing is generally the last timing at which a feed stop mechanism for inhibiting simultaneously feeding of two or more sheets to the printing press can be operated. More specifically, if a mechanical timing for one sheet until feeding of the next sheet is started after the present sheet is gripped by the swing gripper and is fed is assigned as 360.degree. and a feed start timing of a sheet to the printing press is defined as 0.degree., the multiple sheet detection timing is set to be, e.g., 260.degree., i.e., a difference between the 0.degree. timing and, for example, 100.degree. corresponding to the operation time of the feed stop mechanism. That is, the multiple sheet detection timing is the last timing at which the start of sheet feeding can be interrupted. This multiple sheet detection timing is set as the predetermined feed operation timing.
According to this conventional detection method, although simultaneous feeding of two or more sheets detected in the range of 0.degree. to 260.degree. can be detected, simultaneous feeding in a subsequent range cannot be detected. Therefore, simultaneous feeding of two or more sheets in the subsequent range cannot be prevented in advance, resulting in inconvenience.